Conventional projection lenses used for projecting an image onto a display surface are designed with relatively fast optics, often in the range of f/2 to f/2.5, characterized by handling light at large angles. The low f/# values and correspondingly high angular light are due, in large part, to the high etendue light sources that are used, such as various types of very bright arc lamps and similar light sources.
Pixelized spatial light modulators such as LCD modulators and Digital Micromirror Devices (DMDs), such as the DLP (Digital Light Processor) from Texas Instruments, Inc., Dallas, Tex., are electronic light modulation devices of particular interest for digital cinema and other projection applications. These devices can be used with the high-etendue light sources of conventional projection apparatus and various projection apparatus have been designed using these sources with DMD and LCD devices. However, the disadvantages of high-etendue light and the required optical components are constraints to performance and cost for these digitally controlled spatial light modulators, particularly for LCDs, where highly angular light is unfavorable for producing a quality projected image. The recent advent of solid-state laser light sources having very low etendue is particularly advantageous for LCDs as well as for DLP and other types of spatial light modulators. Among benefits of highly coherent laser light are that it allows the use of smaller, slower lens elements, with values in the f/8 range or slower, while still being capable of providing light of sufficient brightness for many projection applications.
There are a number of requirements for projection lens systems used with spatial light modulators that differ from requirements of conventional projection optics. Among these are the need for a long back focal length or working distance, that is, the distance between the last lens surface and the spatial light modulator. Working distances in excess of 2 times the lens focal length are needed in most cases, in order to accommodate a number of optical components used to combine modulated light from different color paths onto an optical axis and, depending on the type of spatial light modulator used, to provide polarization, filtering, and other conditioning and guiding of the light. A long back focal length helps to reduce the angle of illumination that is provided to the spatial light modulator.
Another desirable characteristic for the optical components in a digital projector is telecentricity, so that light across the field of the spatial light modulator has the same narrow range of angles. A high level of color correction and low distortion are also desirable. In order to provide lens designs that meet all of these requirements, designers have primarily concentrated on lens systems that are optimized for the particular projection environment, designed for a screen or other display surface of a particular size and positioned at a given distance from the projector. Some examples of projection optics for digital projectors are given in U.S. Pat. No. 5,870,228 entitled “Projection Lenses Having Large Back Focal Length to Focal Length Ratios” to Kreitzer et al. and U.S. Pat. No. RE39,424 entitled “Telecentric Lens Systems for Forming an Image of an Object Composed of Pixels” to Moskovich. A variable zoom projection apparatus is described in U.S. Pat. No. 6,417,791 entitled “Zoom Projection Lens Having a Lens Correction Unit” to Moskovich.
Lens attachments, familiar to camera users, have been used for providing various changes to the effective focal length and field of view of these image capture devices in forming images onto a film or onto a digital receiver. However, with the exception of specialized anamorphic lens attachments that adjust the image aspect ratio of film media for the projection screen in one dimension only, lens attachments have traditionally been avoided by projection lens designers for a number of reasons. In conventional projector design, the difficulties of controlling aberration and distortion with light at low f/# values make it highly undesirable to add further lens elements once an existing design is optimized for a certain range of angles and projection distance. In general, then, conventional projection optics design approaches are primarily directed to projection systems with fixed focus and field of view, so that, as a result, a projector is designed for a relatively narrow range of applications.